a) Field of the Invention
The present invention relates to an electronic camera, and more particularly to an electronic camera capable of obtaining moving image data and still image data by using a MOS (metal oxide semiconductor) type solid-state image pickup device.
b) Description of the Related Art
Since mass production techniques of CCD (charge coupled device) were established, apparatus utilizing a CCD type solid-state image pickup device as a line sensor or an area sensor has prevailed rapidly.
MOS type solid-state image pickup devices having a smaller consumption power than CCD type solid-state image pickup devices are under development with the advent of wide spreading personal digital assistants and the like. The consumption power of a MOS type solid-state image pickup device can be reduced, for example, about ⅕ to 1/10 of that of a CCD type solid-state image pickup device.
A MOS type solid-state image pickup device has: a semiconductor substrate; a number of photoelectric conversion elements (e.g., photodiodes) formed in one surface of the semiconductor substrate in a matrix shape along a plurality of rows and columns; a switching circuit formed at each photoelectric conversion element; an output signal line disposed for each photoelectric conversion column in the vicinity thereof; and the like. It is desired that the output signal line has a low electrical resistance, and this signal line is usually made of metal material.
In this specification, of the disposal directions of a number of photoelectric conversion elements formed in a matrix shape along a plurality of rows and columns, the disposal direction extending in the same direction as that of the output signal line is called a “photoelectric conversion element column direction” and the disposal of photoelectric conversion elements in this direction is called a “photoelectric conversion element column”. The disposal direction crossing the photoelectric conversion element column is called a “photoelectric conversion element row direction” and the disposal of photoelectric conversion elements in this direction is called a “photoelectric conversion element row”. If an output signal line is disposed in a zigzag way, the direction of a line approximating the overall extension direction of the output signal line is the extension direction of the output signal line.
Several MOS type solid-state image pickup devices having different structures are known. In one type of a MOS type solid-state image pickup device, each switching circuit includes an output transistor and a reset transistor. In the specification, the MOS type solid-state image pickup device is called where applicable a “MOS type solid-state image pickup device I”.
In each switching circuit of the MOS type solid-state image pickup device I, the control terminal (gate electrode) of an output transistor is electrically connected to the photoelectric conversion element, and a load resistor is provided for each output signal line. As a voltage representing an amount of charge accumulated in the photoelectric conversion element is applied to the control terminal of the output transistor, an output signal is generated on the corresponding output signal line. This output signal is detected and converted to an analog voltage signal. The analog voltage signal is directly used as an image signal or converted into a digital signal to be used as the image signal.
Charges accumulated in the photoelectric conversion element do not disappear. It is therefore necessary that prior to generating the next output signal, the charge accumulated and held in the photoelectric conversion element even after the output signal is generated, are required to be drained to a predetermined line or the like. Draining the charge in the photoelectric conversion element is controlled by using the reset transistor. One reset transistor is provided for each photoelectric conversion element.
In an electronic camera having a moving image mode for outputting moving image data and a still image mode for outputting still image data, image pickup is generally and regularly performed in the moving image mode, and only when necessary, image pickup is performed in the still image mode. During the image pickup, an image signal read period and a vertical blanking period are alternately repeated.
In an electronic camera having a MOS type solid-state image pickup device, during each image signal read period, output signals necessary for image data of one frame are generated by the MOS type solid-state image pickup device.
If the MOS type solid-state image pickup device is the MOS type solid-state image pickup device I, output signals necessary for image data of one frame are sequentially generated on the output signal lines in the unit of a photoelectric conversion element row. The operation of the output transistors is controlled in the unit of the photoelectric conversion element row. A row selection signal line is provided for each photoelectric conversion element row. A row selection signal is supplied to the corresponding switching circuits via the row selection signal line to control the operation of the switching circuits.
In this specification, an operation of making the MOS type solid-state image pickup device generate output signals necessary for image data of one frame is called an “image signal read operation”. One image signal read operation is performed during one image signal read period. Generally, when the image signal read period starts, the image signal read operation starts, and when the image signal read period ends, the image signal read operation ends. The length of one image signal read period is, for example, about 1/60 sec to 1/30 sec.
If a MOS type solid-state image pickup device is the MOS type solid-state image pickup device I, charge stored in the photoelectric conversion element can be drained by operating the reset transistor. As light continues to be incident upon the photoelectric conversion element, new charge accumulation starts after the charge is drained. An electronic shutter operation can be performed at a desired timing.
The electronic shutter operation is performed by sequentially controlling the operation of reset transistors, for example, in the unit of a photoelectric conversion element row. Charges accumulated in the photoelectric conversion elements are sequentially drained in the unit of a photoelectric conversion element row. A time taken to perform the electronic shutter operation for all photoelectric conversion element rows is approximately the same as that of the image signal read period. For example, the electronic shutter operation starts at a predetermined timing during the image signal read period and ends at a predetermined timing during the next image signal read period.
An exposure time of each photoelectric conversion element corresponds to a period from when the electronic shutter operation starts to when the next image signal read operation starts.
One reset signal line is provided for each photoelectric conversion element row to control the reset transistors. A reset signal is supplied to the corresponding reset transistors via the reset signal line.
When necessary, at a desired timing between the image signal read operation and the electronic shutter operation to follow, e.g., at a timing when output signals are generated on the output signal lines in the unit of a photoelectric conversion element row, charges accumulated in the photoelectric conversion elements from which the output signals were generated are drained to predetermined line or the like. An operation of generating output signals on the output signal lines and an operation of draining charges accumulating in the photoelectric conversion elements are performed in this order for each photoelectric conversion element row. At a predetermined timing thereafter, the electronic shutter operation is performed.
A readout row-shifter for supplying the row selection signal to each row selection signal line at a predetermined timing is formed in many cases on the same semiconductor substrate as the MOS type solid-state image pickup device. A reset row-shifter for supplying the reset signal to each reset signal line is formed in many cases on the same semiconductor substrate.
If an analog/digital converter (hereinafter abbreviated to “A/D converter”) is provided for each output signal line, a digital signal can be obtained. The A/D converter outputs a digital signal representing an input analog signal, for example, to a buffer memory.
In a MOS type solid-state image pickup device having A/D converters, digital output signals from the A/D converters are used as the image signals.
The operations of each row-shifter, each A/D converter, and buffer memory are controlled by a controller. The controller is formed in many cases on the same semiconductor substrate.
In order to obtain a bright still image of an object in backlight, it is desired to use a strobe or a flash and avoid insufficient exposure of the object. A backlight correction is therefore desired.
Among electronic cameras utilizing a MOS type solid-state image pickup device conducting a row-sequential image signal read operation, electronic cameras having a backlight correction function have not been developed as yet.